. >> a few months ago roberta lou went on facebook and saw an ad about a fat buster. >> if you can take two pills a day, you are in. >> reporter: online she found a ton of them for sale. >> so i clicked on it and i ordered it. >> reporter: without changing her diet or adding exercise. >> i lost my belly fat and ten pounds in two months without even trying. >> reporter: what is she taking? green coffee extract. green coffee extract comes from unroasted or green coffee beans. what is in them? >> caffeine, a well-known stimulant, but the beans also contain chloro genic acid, a substance believed to help boost your metabolism of fats and carbs. 16 subjects lost more than 10% of their body weight in five months, but some experts are voicing strong concerns. >> i worry that we don't even know the right dose to take of coffee extract. >> reporter: dr. mark hellerstein is a scientist at uc berkeley and said the study is too small to draw any conclusions and warns of consequences. >> people want to take it now, i would be careful. we don't know if -- about toxicity. >> or what is in them. they d